Charter for Working Group

The LEDBAT WG is chartered to standardize a congestion control mechanism that should saturate the bottleneck, maintain low delay, and yield to standard TCP.

Applications that transmit large amounts of data for a long time with congestion-limited TCP, but without AQM, fill the buffer at the head of the bottleneck link. With FIFO queue, this increases the delay experienced by other applications. With buffer of one RTT, the delay doubles. In some cases, the extra delay may be much larger. This is a particularly acute and common case is when P2P applications upload over thin home uplinks: delays in these cases can sometimes be of the order of seconds.

The IETF's standard end-to-end transport protocols have not been designed to minimize the extra delay introduced by them into the network. TCP, as a side effect of filling the buffer until it experiences drop-tail loss, effectively maximizes the delay. While this works well for applications that are not delay-sensitive, it harms interactive applications that share the same bottleneck. VoIP and games are particularly affected, but even web browsing may become problematic.

LEDBAT is a transport-area WG that will focus on broadly applicable techniques that allow large amounts of data to be consistently transmitted without substantially affecting the delays experienced by other users and applications.

The WG will work on the following:

(1) An experimental congestion control algorithm for less-than-best-effort "background" transmissions, i.e., an algorithm that attempts to scavenge otherwise idle bandwidth for its transmissions in a way that minimizes interference with regular best-effort traffic.

Desired features of such an algorithm are:

* saturate the bottleneck

* eliminate long standing queues and thus keep delay low when no other traffic is present

* quickly yield to traffic sharing the same bottleneck queue that uses standard TCP congestion control

* add little to the queueing delays induced by TCP traffic

* operate well in networks with FIFO queueing with drop-tail discipline

* be deployable for popular applications that currently comprise noticeable fractions of Internet traffic

Application of this algorithm to existing transport protocols (TCP, SCTP, DCCP) is expected to occur in the working groups that maintain those protocols.

Once experience is gained with this congestion control algorithm on the Internet, the WG will consider if it is appropriate to ask the IESG to advance the document as a Proposed Standard.

(2) A document that clarifies the current practices of application design and reasons behind them and discusses the tradeoffs surrounding the use of many concurrent TCP connections to one destination and/or to different destinations.

Applications routinely open multiple TCP connections. For example, P2P applications maintain connections to a number of different peers while web browsers perform concurrent downloads from the same web server. Application designers pursue different goals when doing so: P2P apps need to maintain a well-connected mesh in the swarm while web browsers mainly use multiple connections to parallelize requests that involve application latency on the web server side. The IETF transport area community is concerned about this practice, because standard Internet congestion control results in different transport connections sharing bottleneck capacity. When an application uses several non-rate-limited transport connections to transfer through a bottleneck, it may obtain a larger fraction of the bottleneck than if it had used fewer connections. Although capacity is the most commonly considered bottleneck resource, middlebox state table entries are also an important resource for an end system communication. Other resource types may exist, and the guidelines are expected to comprehensively discuss them.

Applications use a variety of techniques to mitigate these concerns. These techniques have not always been reviewed by the IETF and their interaction with TCP dynamics is poorly understood. The WG will document the known techniques, discussing the consequences and, where appropriate, provide guidance to application designers.

Milestones

Oct 2009

Submit 'Multiple Transport Connections in Applications Design' to the IESG for consideration as an Informational RFC

Oct 2009

Submit 'Low Extra Delay Background Transport (LEDBAT)' to the IESG for consideration as an Experimental RFC

Feb 2010

Submit 'A Survey of Lower-than-Best Effort Transport Protocols' to the IESG for consideration as an Informational RFC